“STAINLESS STEEL” is a generic term covering a large group of alloys known for their resistance to staining, corrosion and pitting and like a common soup, the ingredients can be changed to affect the final performance or result. Stainless steel generally defines a chromium content in excess of 11% and less than 30%. The base of steel is iron, which means many alloys will be MAGNETIC .
They are not indestructible and the key to obtaining optimum results from stainless steel in marine applications is the use of the appropriate stainless steel for the specific application. The design of Fleming systems has taken careful account of the properties of particular grades of stainless steel to produce a series of wind vanes that are materially better than any of our competitors.
Austenitic grades are the most widely used in the marine field and accounts for nearly half of all stainless steel sold. The term austenitic implies carbon in solid solution. The structure is “non magnetic” and can be retained at room temperature by appropriate alloying. The most common austenite retainer is nickel so the traditional and familiar austenitic stainless steels have a composition that contains sufficient chromium to offer corrosion resistance together with nickel to ensure austenitic at room temperature and below.
In summary, the family of stainless steels can be split into five main groups: Martensitic, Ferritic, Austenitic, Duplex and Precipitation Hardening steels. Ninety five per cent of all stainless steel sales are are either ferritic or conventional austenitic steels.
The basic austenitic composition is the familiar 18% chromium and 8% nickel alloy, this grade is known as 316 or 18/8 worldwide. Both alloys are increased to improve corrosion resistance and additional elements such as molybdenum will further enhance corrosion resistance. 316 or 18/8 has 2% added molybdenum to improve its corrosive qualities.
Fleming uses this particular alloy exclusively in shafting and tubing.
The Family Tree of Stainless Steels
The Closing Branch and the Fleming Difference; Duplex Stainless Steel – a blend of austenitic and ferritic steels
The 2205 duplex stainless steel and the precipitation hardened 17-4 alloy were developed for specific purposes and have been available since the 1980’s. Duplex stainless steels contain between 18 and 28% chromium, 4.5 to 8% nickel and 2.5 to 4% molybdenum. The relatively high content of nickel is sufficient to create a fully austenitic structure. When this alloy is combined with a ferritic stainless steel – hence the description duplex – a truly superior material is obtained.
Duplex stainless steels have some excellent properties for the marine environment:
High resistance to stress, corrosion, cracking and increases resistance to chloride ion attack
The yield strength of Duplex Steels is about two to three times higher than that of austenitic steels
The higher yield strength results in a stronger, lighter structure improving strength/weight rat
Fleming makes use of this superior form of stainless steel alloy in both castings and shafting. The very high proof strength is achieved from a smaller grain size in a two phase microstructure. This small grain size prevents grain growth and increases strength and toughness.
17.4PH OR STAINLESS W (630)
In certain castings where increased tensile strength is desired, Fleming uses this alloy to further reduce unit weight. Tensile strength increases from 2205 (680-900MPA) to 17.4PH (1035MPA). These are approximate values.
What does that mean for clients?
The new alloys have improved the corrosive resistance and long term durability of Fleming Systems with a weight comparable to Aluminium or stainless steel tubular construction.